Process of manufacturing ferricyanides



Patented May 9, 1933 UNITED STATES WALTER KLEMPT, OF DOR'IMUND-EVIlSl'G, GERMANY :PnooEss on MANUFACTURING nn'nnioynmnns No Drawing. Application filed June 29, 192$,Seria1 No. 374,957,:1ndin Germany my 5, 1:528. '7 l 5 process consisting in treating the ferrocyanides together with carbonic acid and air for some time under pressure and at anincreased temperature in an autoclave. It has been found, however, that wheneftecting the oxidation under pressure in the presence of oxygen or air always a certain portion of the ferrocyanide remains and can be converted into ferricyanide only with considerable clifficulty. The percentage of the ferrocyanide remaining towards the end of the procedure is different according to the kind of the ferrocyanic salt employed. When working, for

instance, with ferrocyanide of calcium an oxidation up to from 95 to 97% can in general be attained without particular difticulty.

In order to obtain the ferricyanide from a solution the process can be carried out in general in this way that the solution is subjected to a fractionated crystallization. But in this procedure the ditliculty is encountered that only a small portion of the ferricyanide can be separated, this small portion being, unfortunately, impurified with adhering fer rocyanide, whereas a large portion of the ferricyanide remains mixed with the ferro cyanide in the mother-lye, which latter must then again be subjected to an oxidation under pressure.

It has, furthermore, been foundthat the output in pure ferricyanide can be render-ed quantitative in a simple manner by combining the process of oxidizing under pressure with the known electrolytic oxidation of the ferrocyanides. It is in this combination an advantage that the final product which consists solely of pure ferricyanide and is obtained in dissolved state can be obtained in solid state, as solid ferricyanide, merely by concentration, preferably under vacuo, to the commencement of the crystallization, the entire amount of the dissolved ferricyanide being then obtained perfectly free from ferrocyanides.

The current output decreases, as is known, with increasing oxidation of the solution.

If only small amounts of the ferricyanide are to be oxidized electrolytically in the presence of a large amount of ferricyanide, the current output will in general be only small the subsidiary reactions, especially the arising of free ferricyanic hydracid and, in connection therewith, an alkalization of the cathode space, entail an increased loss of current, as well as of output. In" order to conduct the electrolytic oxidation in such a manner'that secondary subsidiary reactions are obviated possibly much, the anode space must continually be kept neutral or, at the highest, moderately alkaline so that the formation of freehydro-ferricyanidacid is obviated. y

It has been proved advantageous to make use of the calcium salt for the transformation of ferrocyanide into ferricyanide. It has been found that under certain conditions the CaO passing over into the cathode space separates on the cathode in such a solid statethat it can be transferred to the anode space simply by exchanging the electrodes, the 0210 being then used in the anode space for the neutralization of the free acid.

While in the anode space the electrode is freed from the adhering lime, GaO collects meanwhile anew on the cathode, and can then be utilized anew in the anode space after changing over of the conducting connections. It is, with this manner of operation, suitable for the purpose in view to fill the oath ode space with lime water.-

The combination of the oxidation under pressure with the electrolytic oxidation of the ferrocyanides, as well as the employment of the calcium ferrocyanide, presents, besides, the advantage that also the calcium carbonate mud arising during the oxidation under pressure can be utilized in the anode space for the'neutralizationof the free acid.

Of course, instead of the soluble Ca-fer' rocyanide also double-salts containing Ga,

such as ammonium-cal ium-ferrocyanide, po-

tassium-calcium-ferrocyanide or sodium-calcium-ferrocyanide, and so' on, can-beused, if, in the case of the use of such insoluble Ca-salts, sutficient movement thereof in the anode space is provided for. i I 1 I am now giving two examples as to how thls improved process can be carried out:

First example A 15% -solution of calcium ferrocyanide is treated with a mixture of air and carbonic acid in an autoclave under a pressure of about 50 atm. and at a temperature of about 100 C. Said mixture is composed of about 80% of air and about 20% of carbonic acid. In an hour, the ferrocyanide present is being oxidized to ferricyanic calcium, except asmall rest amounting to about 5% of the original ferrocyanide. The solution is withdrawn from theautoc'lave in which the pressure is maintained, and is divided onto the anode spaces of the battery of electrolyzers, the cathode spaces of which are filled with lime water.

The electrodes consist of sheet-nickel. On the sheet-nickel plates standing in the oathode space C210 is separated during the electrolytic oxidation which latter is carried out preferably with a slight density of the current. The cathode sheet-metal plate is, at the next electrolytic oxidation, used in the anode space so that there is always an excess of lime. The strength of the current used in the electrolytic oxidation amounts preferably to 0.005 amp. per square centi meter of the electrode surface. After the ele'ctrolyze has been finished in the anode space no trace of ferrocyanic calcium can be proved to be still present.-

In order to obtain from the Ca-ferricyanide' other ferricyanides, for instance the potassium-ferricyanide, the solution leaving the electrolyzer is converted with an equivalent amount of potassium carbonate. After the separated-calcium carbonate has been filtered oil, the solution is concentrated in known manner under vacuo in order to obtain the pure potassium-ferricyanide.

Second example A 15,%-so1ution of calcium-ferrocyanide is conveyed from a store receptacle into a washing space able to stand a high internal pressure. In this space said solution trickles counter to a mixture of oxygen and carbonic acidwhich is compressed to about 50 atm. and contains about 20% of the carbonic acid. The reaction temperature is maintained on about from 120 to 150 C. After the solution has streamed one time through the reaction space or tower in which said high pressure is continually maintained it can be withdrawn; it is,- except the few percents of ferrocyanide which must be oxidized electrolytically, free from ferroc'yanide'. The amount of air or oxygen and carbonic acid consumed is again re-conducted to the current of gas.

I claim: 1. The process of converting ferrocyanide in solution to ferricyanide which comprises treating the solution of ferrocyanide in a closed vessel under a pressure above atmospheric and at a high temperature with a gaseous mixture containing free oxygen and carbon dioxide until the greater part of the ferrocyanide is converted into ferricyanide, then withdrawing the solution from the said vessel and effecting the conversion of the remaining ferrocyanide to ferricyanide by electrolysis of the solution.

2. The process of converting calcium ferrocyanide in solution to calcium ferricyanide which comprises treating the solution of calcium ferrocyanide in a closed vesselin the presence of air and carbon dioxide until an equilibrium is obtained and the greater portion of the calcium ferrocyanide has been converted to calcium ferricyanide, then withdrawing the solution from the vessel and effecting the conversion of the remainderv of the calcium ferrocyanide to calcium ferricyanide by electrolysis. V

'3. In the electrolysis of a solution containing calcium ferrocyanide for the conversion of said calcium ferrocyanide to calcium ferricyanide, the step which comprises employing'as the anode in said electrolysis an electrode which has been employed as cathode in a previous similar electrolysis and which has thereby been coated with calcium oxide.

4. The process of converting calcium ferrocyanide in solution to calcium ferricyanide which comprises subjecting the solution of calcium ferrocyanide to a pressure of not less than 50 atmospheres and a temperature of not less than 100 C. in the presence of air and carbon dioxide until an equilibrium is reached and the greater part of the calcium ferrocyanide has been converted to calcium ferricyanide, then transferring the solution to the anode of an electrolytic vessel and effectin the conversion of the remaining calcium errocyanide to calcium ferricyanide by electrolysis, the anode employed being an electrode employed as cathode in a previous similar operation and thereby coated with calcium oxide. I

In testimony whereof I affix my signature.

WALTER KLEMPT. 

